The invention relates to a power supply unit for electronic flash, and more particularly, to such unit including a plurality of DC-DC converters which efficiently feed a main capacitor of a large size electronic flash.
As is well recognized, an electronic flash is usually constructed as a portable unit to enable a flash photography outdoors, and is hence fed from a battery which is either contained within or externally connected to the electronic flash. A battery has an electromotive force of a magnitude which is insufficient to charge the main capacitor to a desired level, and hence a booster which converts the low voltage output from the battery to a higher level is normally provided as either an internal or an external component of the electronic flash.
FIG. 1 shows one example of a conventional power supply unit for electronic flash. As shown, the unit includes a d.c. source E1 comprising a plurality of series connected dry cells to provide a given voltage. A power switch SW1 is connected in series with the source E1 and is connected, when closed, to feed a DC-DC converter DCC.sub.0 which initiates a self-excited oscillation to convert the low voltage output from the source E1 to a higher level. The converter DCC.sub.0 includes a step-up transformer T1 having a primary and a secondary winding P and S. An oscillation transistor Q1 of PNP type has its base connected to one end of the secondary winding S, and has its collector connected through resistor R1 to the base of a main transistor Q2 of NPN type. A series combination of resistor R2 and capacitor C2 is connected across the source E1 through the switch SW1. A rectifier diode D1 has its anode connected to the other end of the secondary winding S. The base of the transistor Q1 is connected to the junction between the resistor R2 and capacitor C2, and has its emitter connected to a common line l.sub.0 which is connected through the power switch SW1 to the positive terminal of the source E1. The emitter of the main transistor Q2 is connected to the negative terminal of the source E1. The primary winding P has its one end connected to the common line l.sub.0 and its other end connected to the collector of the main transistor Q2.
A supply line l.sub.1 is connected to the cathode of the diode D1 to feed an operating voltage for an associated electronic flash. The lines l.sub.1 and l.sub.0 are connected to a pair of output terminals J1 and J2 of the power supply unit, across which is connected a flashlight emission circuit FIC.sub.1 including a main capacitor CM1. The flashlight emission circuit FIC.sub.1 operates to generate flashlight for taking a picture, by causing a discharge through a flash discharge tube FL1 of the main capacitor CM1 which is previously charged to a high voltage by the power supply unit. The emission circuit FIC.sub.1 comprises a trigger switch SW2, a trigger capacitor C1, and a trigger transformer T2, in addition to the flash discharge tube FL1 and the main capacitor CM1. Specifically, the main capacitor CM1 is connected across the pair of output terminals J1 and J2. Also connected across the output terminals are a series circuit including a resistor R3 and a neon lamp Nel which indicates the completion of a charging operation, and another series circuit including a resistor R4 and the trigger switch SW2. Also the flash discharge tube FL1 is connected across the output terminals. The junction between the resistor R4 and the trigger switch SW2 is connected to one end of the trigger capacitor C1, the other end of which is connected to one end of a primary winding of the transformer T2. The other end of the primary winding is connected to the line l.sub.0 and is also connected to one end of the secondary winding, the other end of which is connected to a trigger electrode FL1a of the flash discharge tube FL1.
In operation, when the power switch SW1 is closed, the oscillation transistor Q1 begins its oscillating operation as it is fed from the source E1, thus activating the converter DCC.sub.0. Accordingly, the combination of the step-up transformer T1 and the diode D1 develops a high d.c. voltage across the output terminals J1 and J2, which charges the main capacitor CM1 and the trigger capacitor C1 in a given manner. As a shutter release of a photographic camera is operated, the trigger circuit is activated in synchronism therewith, whereby the main capacitor CM1 discharges through the discharge tube FL1 to emit flashlight.
The power supply unit shown in FIG. 1 including the converter DCC.sub.0 is designed to be assembled into an electronic flash. However, for use with an electronic flash of a relatively large size, an arrangement as shown in FIGS. 2 and 3 may be used which permits external power supply unit or units to be used in addition to the internally housed power supply unit.
An example of a conventional electronic flash of a relatively large size ST is shown in FIG. 2, and essentially comprises an emission control circuit A.sub.0 which is activated by the trigger switch SW2 to cause the emission of flashlight from the flash discharge tube FL1, an emission adjusting circuit B.sub.0 which is adapted to determine reflected light from an object being photographed when the flashlight is emitted to cease the emission of flashlight from the discharge tube FL1 by controlling the operation of the emission control circuit A.sub.0, an external control circuit E.sub.0 and the main capacitor CM1. These circuits and the main capacitor CM1 are connected across the output terminals J1 and J2 of the power supply unit as shown, and an internally housed power supply unit C.sub.0 which is constructed in the a manner similar to that shown in FIG. 1 is connected across these output terminals through the power switch SW1. An external power supply unit D.sub.0 may be connected through electrical cords F.sub.0 with the electronic flash ST which consists of the combination of the flashlight emission circuit and the internally housed power supply unit, as illustrated in FIGS. 2 and 3. As shown in FIG. 3, the external power supply unit D.sub.0 includes a plurality of batteries D.sub.01 each having an increased capacity and a booster circuit D.sub.02 having a DC-DC converter. The output terminals of the unit D.sub.0 are adapted for connection with the output terminals J1, J2 of the power supply unit as shown in FIG. 2. When the external unit D.sub.0 is used, the power switch SW1 is opened to disable the internal unit C.sub.0.
A portable electronic flash of a large size which may be used by a press photographer includes a main capacitor of an increased capacity so that a higher guide number can be used. To permit a rapid charging of such main capacitor, the electronic flash is associated with a power supply unit capable of supplying an increased output. A conventional power supply unit which is designed for use with an electronic flash of a large size has a circuit arrangement as illustrated in FIGS. 1 to 3 in a manner similar to that used for an electronic flash of a small size. This involves the following disadvantages:
(1) An increased current drain from the battery causes a rapid reduction in the discharge rate of the battery, preventing an efficient use of the battery.
FIG. 4 graphically shows the relationship between the discharge current and the discharge capacity of a nickel-cadmium battery, or a change in the discharge rate. It will be seen that the greater the discharge current, the less the discharge capacity. In this Figure, the unit "C" used to denote the discharge current represents a nominal capacity (one hour rate). Specifically, considering a nickel-cadmium battery having a nominal capacity of 500 mAh, when the battery is caused to discharge continuously with a current of 50 mA which is equal to one-tenth the nominal capacity (one hour rate), this is referred to as 0.1 C discharge. A corresponding discharge capacity is designated as 100%. Accordingly, 2.0 C, for example, means a discharge with a current which is equal to twice the nominal capacity (one hour rate). The electrical capacity drawn from the battery at a variety of values of discharge current is shown as a discharge capacity (in percentage) in FIG. 4 as compared with 0.1 C discharge.
It will be seen from FIG. 4 that when the discharge current is equal to 3.0 C, the discharge capacity will be reduced to a value slightly less than 80% of the discharge current for 0.1 C, and the discharge capacity will be reduced to a value slightly greater than 70% for the discharge current of 4.0 C. A conventional power supply unit for use with an electronic flash of a large scale is usually operated at discharge current of 10 to 20 C. Accordingly, the discharge capacity will be on the order of one-third or less than that available in the 0.1 C discharge, resulting in a considerable reduction in the utilization efficiency of the battery. The discharge current from the battery on the order of 10 to 20 C is required as a result of an increased current flow through the primary side of the DC-DC converter in order to charge the main capacitor with an increased magnitude of current to thereby reduce the required charging time. The greater the discharge current from the battery and the longer the duration of the discharge, the less the discharge capacity will be, due to the utilization of the active material in the plates of the battery at a reduced efficiency, increasing the internal loss.
(2) The overall size of the power supply unit increases, presenting an inconvenience in its portable use.
A conventional power supply unit for use with an electronic flash of a large size has a circuit arrangement as shown in FIG. 1. Accordingly, components used in the circuit arrangement must be of sizes which are commensurate with the capacity of the main capacitor CM1 to be charged. In particular, the transistor Q2 which represents the main element in the oscillating operation must pass a current flow through the primary winding of the step-up transformer T1 and hence must have an increased capacity. It is associated with a heat dissipating plate of an increased size to accommodate for an increased amount of heat produced during the self-excited oscillation. In order to avoid adverse influence of the heat produced by the transistor Q2, a certain space must be secured between the transistor and its peripheral circuit in actual implementation.
A conventional power supply unit for use with an electronic flash of a large size is generally designed to operate with a supply voltage of 12 V (1.5 V.times.8) which is twice the operating voltage of 6 V (1.5 V.times.4) used for a normal electronic flash of a small size. As the operating voltage is stepped up, a greater current tends to flow through the primary size of the converter. Unless the magnitude of the current is suppressed to a degree, the heat produced by the transistor will be excessively high, causing an overloading and a reduced efficiency of the battery. To accommodate for this, in the prior art power supply unit designed for use with an electronic flash of a large size, the primary winding P of the step-up transformer T1 has an increased number of turns, with a corresponding increase in the number of turns of the secondary winding, thus increasing the resistance of the windings in an attempt to suppress the current drain from the source E1 in order to prevent a reduction in the efficiency. This resulted in an increased size of the step-up transformer T1 and hence the power supply unit.